Since its inception in 1965, this project has been involved in the synthesis and evaluation of potential radiodiagnostics. The synthetic program has been based on two approaches: 1) Pharmacological Approach--labelling of drugs which have been reported or would be expected to have a predilection for a particular organ or tumor, and 2) Biochemical Approach--labelling of naturally-occurring compounds or their precursors which are known to be biosynthesized and/or stored in certain organs or tumors. In our design of organ tumor imaging agents we will continue to focus much of our attention on diseases of the endocrine system. To achieve our goal, the chemical effort will deal primarily with steroids and amine derivatives and their radiolabelled counterparts. In general, compounds will be radiolabelled with gamma-emitting nuclides such as 125I, 131I, 82Br, 77Br, and 18F. Once the synthesis of the radiolabelled compound has been achieved, tissue distribution studies will be performed in normal or tumored animals (usually mice and rats). Those agents displaying selective localization in a target organ or tumor will then be studied in larger animals (usually dogs). Although current research emphasizes the development of imaging agents for tumors associated with adrenal, prostate, ovary, pituitary, and mammary gland, agents which display a potential for photoscanning other organs or tumors will not be ignored.